The invention concerns the joint production, from heavy petroleum cuts, of middle distillates and high viscosity oil bases, ie., oils with viscosity indices (VI) of between 95 and 150, more particularly between 120 and 140.
The boiling points of the feedstocks are more than 380.degree. C., for example vacuum distillates, deasphalted oils or mixtures thereof.
The Institut Francais du Petrole has been developing processes for the production of oil bases from these feedstocks for a long time, whether by extraction (using furrural, for example) or by hydroraffination. In the latter case, amorphous catalysts containing nickel and molybdenum supported on alumina or an aluminosilicate are used (French patent FR-A-1 465 372).
A two step process using two different amorphous catalysts is also known. Thus in U.S. Pat. No. US-A-3 642 612, the feedstock is treated in the presence of hydrogen using a first catalyst containing metals from groups VI and VIII deposited on a slightly acid support (alumina) then using a second catalyst also containing metals from groups VI and VIII but deposited on a more acidic support (silica-alumina).
We have produced oil bases with at least the same VIs as those produced by a process using amorphous catalysts, but having higher viscosities (with respect to a process using amorphous catalysts) for isoconversion to distillates.
In other words, this process allows more middle distillate production while conserving the characteristics of similar oils.
We have developed a flexible process which can be adapted for a variety of cuts and which allows the refiner to control conversion and viscosity.